1. Field of the Invention
The present invention relates to friction stir welding apparatus wherein a rotating pin tool is plunged into workpieces to create a weld, and, more particularly, to a penetration control system for the pin tool of such a friction stir welding, apparatus.
2. Background of the Invention
A friction stir weld machine is used to join two metal workpieces together. The two metal workpieces to be joined are positioned in a butt join configuration and clamped into this position. As indicated above, a friction stir weld is formed by plunging a rotating shouldered pin tool into the two closely fitting metal workpiece faces until the shoulder of the pin tool contacts the work surface. The initial pro be depth of the distal end of the pin of the pin tool is less than the required weld depth, and rotation of the pin within the workpiece provides frictional heating of the metal and produces a plasticized column of metal around the distal, probe end of the pin. The pin tool is moved in the direction of welding and, as this happens, the leading face of the probe end of the pin crushes the plasticized material and forces this material to the back of the pin. At the same time, the pin tool applies a substantial downwardly acting mechanical forging force. This force consolidates the weld metal into the weld joint at the trailing end of the tool.
Currently, pin tools for friction stir welding are of a fixed pin length. In commonly assigned U.S. Pat. No. 5,893,507 (Ding et al), the subject matter of which his hereby incorporated by reference, there is disclosed an automatically adjustable pin tool for friction stir welding wherein the pin tool automatically adjusts for welding materials of varying thicknesses and wherein the pin can be incrementally withdrawn from the workpiece to thereby eliminate any keyhole or crater in the weld.
In friction stir welding, it is important to control the plunge depth of the pin of the pin tool into the weld joint in order to produce the desired precision weld. This control is not possible with current systems. Mechanical systems wherein movement of the pin is controlled using a manual crank wheel do not provide sufficient control precision.
In accordance with the invention, a control system is provided for precisely controlling the depth or location of the distal end of the stirring pin of a rotating pin tool of a friction stir welding machine or apparatus so as to control the penetration of the pin into the workpiece.
According to the invention, there is provided, in a friction stir welding apparatus comprising a pin tool including a shoulder and a rotating pin extending outwardly from the shoulder of the pin tool, a control system for controlling penetration of the pin tool in a workpiece comprising contacting workpiece members which are to be joined by stir welding and which are mounted on a support anvil, the control system comprising: a pin length controller for controlling pin length relative to the shoulder and for producing a corresponding pin length signal; a pin force sensor for sensing the force being exerted on the pin during welding and for producing a corresponding actual pin force signal; a sensor for sensing a parameter related to the position of the shoulder of the pin tool; and for generating a shoulder position signal based thereon; and a control unit for receiving said pin length signal, said pin force signal, and said shoulder position signal, and for, based on said signals, controlling the pin length controller so as to control pin penetration ,into the workpiece.
The apparatus preferably further comprises a workpiece standoff sensor for sensing the standoff distance between the workpiece and the standoff sensor and for supplying a corresponding standoff signal to said control unit.
Advantageously, the control unit further receives a signal related to a weld schedule for the welding apparatus and further controls the pin length controller based thereon.
Preferably, the apparatus further comprises a probe controller for controlling a probe extending outwardly from said pin, for sensing a probe parameter related to the distance between the probe and the supporting anvil and for supplying a corresponding probe signal to said control unit. Advantageously, the probe parameter comprises probe force. Alternatively, or in addition, the probe parameter comprises probe position.
In a preferred embodiment, the shoulder position parameter comprises shoulder force. Alternatively, or in addition, the parameter comprises depth of shoulder penetration.
Preferably, the apparatus further comprises an actuator for moving the pin relative to the shoulder and the system further comprises a linear encoder for sensing the actual length of the pin, and the pin controller determines the difference between the actual pin length and a desired pin length and controls pin position based on this difference.
Advantageously, the control system further comprises a motion controller for controlling driving of the actuator, and the actuator includes an incremental encoder for supplying an actual velocity signal to the motion controller, the motion controller further receiving the pin length signal from the linear encoder.
The apparatus typically includes adjustable pin tool gearing driven by the actuator for determining the pin length and, preferably, the control system further comprises limit switches associated with the pin tool gearing for supplying limit signals to the motion controller.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.